Introduce bare else clause in if exprs

With a non-passing test

schala-lang/language/src/ast.rs

@@ -239,6 +239,7 @@ pub struct GuardArm {
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum Guard {
+  None,
   Pat(Pattern),
   HalfExpr(HalfExpr)
 }

schala-lang/language/src/parsing.rs

@@ -122,7 +122,7 @@
 //! simple_pattern_match := pattern "then" conditional
 //! else_clause := ε | "else" expr_or_block
 //! guard_block := "{" (guard_arm, ",")* "}"
-//! guard_arm := guard "then" expr_or_block
+//! guard_arm := guard "then" expr_or_block | "else" expr_or_block
 //! guard := "is" pattern | (operator)+ precedence_expr
 //! ```
 //!
@@ -893,10 +893,16 @@ impl Parser {
 
   #[recursive_descent_method]
   fn guard_arm(&mut self) -> ParseResult<GuardArm> {
-    let guard = self.guard()?;
-    expect!(self, Keyword(Kw::Then));
-    let body = self.expr_or_block()?;
-    Ok(GuardArm { guard, body })
+    if let Keyword(Kw::Else) = self.token_handler.peek_kind() {
+      self.token_handler.next();
+      let body = self.expr_or_block()?;
+      Ok(GuardArm { guard: Guard::None, body })
+    } else {
+      let guard = self.guard()?;
+      expect!(self, Keyword(Kw::Then));
+      let body = self.expr_or_block()?;
+      Ok(GuardArm { guard, body })
+    }
   }
 
   #[recursive_descent_method]

schala-lang/language/src/parsing/test.rs

@@ -4,7 +4,7 @@ use std::str::FromStr;
 
 use super::tokenize;
 use super::ParseResult;
-use crate::ast::{ItemIdStore, AST, Expression, Statement, StatementKind, IfExpressionBody, Discriminator, Pattern, PatternLiteral, TypeBody, Enumerator, ForBody, InvocationArgument, FormalParam, PrefixOp, BinOp, QualifiedName, ImportSpecifier, ImportedNames};
+use crate::ast::{ItemIdStore, AST, Expression, Statement, StatementKind, IfExpressionBody, Discriminator, Pattern, PatternLiteral, TypeBody, Enumerator, ForBody, InvocationArgument, FormalParam, PrefixOp, BinOp, QualifiedName, ImportSpecifier, ImportedNames, GuardArm, Guard};
 use super::Declaration::*;
 use super::Signature;
 use super::TypeIdentifier::*;
@@ -761,3 +761,27 @@ fn imports_4() {
   }
 }
 
+
+#[test]
+fn if_expr() {
+  parse_test_wrap_ast! {
+    "if x { 1 then 5, else 20 }",
+    exst! {
+      IfExpression {
+        discriminator: bx!(Discriminator::Simple(ex!(s "x"))),
+        body: bx!(IfExpressionBody::GuardList(
+          vec![
+            GuardArm {
+              guard: Guard::Pat(Pattern::Literal(PatternLiteral::NumPattern { neg: false, num: NatLiteral(1)})),
+              body: vec![exst!(s "5")],
+            },
+            GuardArm {
+              guard: Guard::None,
+              body: vec![exst!(s "20")],
+            },
+          ]
+        ))
+      }
+    }
+  }
+}

schala-lang/language/src/reduced_ast.rs

@@ -276,6 +276,16 @@ impl<'a> Reducer<'a> {
         let mut alternatives = vec![];
         for arm in guard_arms {
           match arm.guard {
+            Guard::None => {
+              let item = self.block(&arm.body);
+              let alt = Alternative {
+                item, matchable: Subpattern {
+                  tag: None, subpatterns: vec![],
+                  bound_vars: vec![], guard: None,
+                }
+              };
+              alternatives.push(alt);
+            },
             Guard::Pat(ref p) =>  {
               let item = self.block(&arm.body);
               let alt = p.to_alternative(item, symbol_table);